Ion channels are membrane proteins that control the flow of ions such as K+, Na+, Ca*+, and CI" across the cell membrane. They regulate many biological processes such as the excitation of nerve and muscle cells, the secretion of hormones, and sensory transduction. In humans, they are found in nearly all tissues serving a variety of tasks. Because of their prevalence and importance in the human body, ion channel dysfunction often lies at the heart of a wide range of human pathologies. Ion selectivity, whereby channels only allow the passage of specific ions through their pores while excluding all others, is one of the characteristic properties defining an ion channel. Understanding this process is central to gaining fundamental knowledge about channel-related biological activities and diseases. Even though tremendous progress has been made over the last five years in understanding K+ selectivity, especially with the structure determination of several K+ channels, there is little structural information available for any other cation channels The overall goal of my research is to understand the structural basis of cation channel selectivity. More specifically, my laboratory will focus on studying the selectivity of two groups of cation channels: non specific cation channels, using the NaK channel from Bacillus cereus, a bacterial Na+ and K+ conducting channel that is homologous to the pore of a CNG channel, as a model system; and the prokaryotic voltage- gated Na+ channels. We will use a combination of crystallographic and electrophysiological tools to characterize these channels both structurally and functionally. The proposed research has three specific aims. The first specific aim is the structural and functional study of monovalent cation conduction in the NaK channel. This study will allow us to elucidate the molecular mechanisms underlying ion permeability in NaK, and will also provide crucial insights into understanding the structural basis of ion selectivity in the CNG channel family. Our second specific aim is to study the divalent cation blockage of the NaK channel. This study will elucidate the underlying mechanism of divalent cation blockage in CNG channels, a process of crucial physiological significance, especially to visual transduction. Third, we aim to determine the crystal structure of the ion conduction pore of a prokaryotic voltage-gated Na+ channel. This study will not only allow us to elucidate the structural basis of ion selectivity in Na* channels, but will also shed light on the ion selectivity of Ca2+ channels whose selectivity filter shares high sequence homology to that of Na* channels.